VSEPR Applied (4 things)

When four things are attached to the central atom, as they are in methane, CH₄, the structure becomes distinctly 3 dimensional.

If we imagine that your hands and feet hate each other (but this time allow your feet to move independently, it is tempting to imagine that the best position would be this one:

This would leave your limbs at 90^o angles to each other. However, it is possible to move them slightly further away from each other. Rotating the top half of your body allows you to increase the angles slightly to 109.5^o. In a human, that would look like this:

In a molecule, that looks like this

This structure is called a tetrahedron.  A more common way of drawing this structure rotates it to appear this way.

Since all of the things around the central atom are atoms (as opposed to lone pairs) the structure and shape are the same.

When one of the four things is a lone pair, as in the molecule NH₃, the structure is still a tetrahedron, but the shape is a trigonal pyramid.

When two of the things are lone pairs, as in water, the structure remains tetrahedron, but the shape is bent 109 (the other bent shape).

A note about angles

Five Things